Radiation therapy is often used to treat cancerous tumors within a patient's body. An early diagnostic session is conducted where the physician uses an imaging technique, such as computed tomography (CT) scanning or magnetic resonance imaging (MRI) to study the target area. He or she then decides the ideal placement and volume of the radiation beam(s) with respect to the target area. During the actual treatment, the radiation beams are focused directly at the target area, using the diagnostic studies as a position reference. Precise positioning of the radiation beams insures that most of the radiation contacts the target cells, while also insuring that the healthy cells surrounding the target cells are not affected. Unfortunately, it is often difficult to be certain that radiation beams are optimally positioned with respect to target cells. Often, a smaller total dose of radiation must be used in order to reduce the possibility of damage to healthy cells. The consequence, however, is that the radiation treatment becomes less effective.
In addition, radiotherapy often requires a patient to return for treatment over the course of several days. Repositioning a patient precisely each time can be time-consuming and frustrating.
Over the past decade, many methods have been devised to improve the alignment of radiation beams with the target area of a patient. An early method involves a rigid frame to physically hold in place the part of the patient's body to be treated. In one embodiment for treatment of target areas within a patient's skull, the frame is attached to a floorstand mounted in a Linac (linear accelerator) floor turret. This method is considered generally reliable and accurate, as it fixes the target area rather precisely with respect to the radiation beams. Unfortunately, due to the nature of the frame, it also greatly limits accessibility to the patient's skull. Target areas may be located in the skull where the Linac radiation beams cannot reach. In addition, it is extremely uncomfortable for the patient, who must remain in an awkward position for long periods of time.
Another method involves invasive techniques. U.S. Pat. No. 5,097,839 by Allen describes fiducial implants attached in a pattern to a patient's skull bones, underneath the skin. These implants are then used as fixed references when aligning radiation beams with the target area. These implants are an improvement over rigid frames in that they allow all target areas within a skull to be reached. However, because inserting the fiducial implants into the patient is a surgical procedure itself, the patient must often wait for several days until the radiation treatment. During this time, the target area may grow or otherwise change in shape, rendering inaccurate the early diagnostic analyses taken when the fiducial implants were put in place. In addition, the implants are often disfiguring and painful to the patient.
Another type of invasive technique involves placing tattoos on the patient's skin where the radiation beams are to enter. Although this is less intrusive than the fiducial implants, it has many of the same problems, such as the patient having to wait several days from the time of the tattoo procedure until the radiation treatment, thus giving time for the target area to grow or change shape. In addition, given the nature of tattoos, it is possible they may also change shape.
More recently, non-invasive, non-disfiguring alignment systems have been developed. These typically use signal processing to convert the CT or MRI data of the position of the patient in the diagnostic setting to the position of the patient in the treatment setting. Many of these systems require a large amount of preprocessing, whereby data generated from the diagnostic scan is gathered and manipulated until it is usable in the treatment setting. The preprocessing step can take several days. During treatment, real time images are compared with the preprocessing data and the patient or the radiation therapy beams are adjusted accordingly. Oftentimes, manual adjustment is necessary. Three degrees of freedom, corresponding to one plane, are typically allowed. The patient has greater freedom of movement than in the previously described techniques, but his movement is still confined. These systems are generally accurate, and painless for the patient.
U.S. Pat. No. 5,295,200 by Boyer et al. describes a method of aligning radiation therapy beams using the Fast Fourier Transform (FFT) to compare the position of diagnostic images with the position of treatment images. In this invention, a large amount of complex data must be gathered and processed prior to treatment. Reference images collected during the diagnostic study are later used to position the patient during treatment.
U.S. Pat. No. 5,531,520 by Grimson et al. describes a method of image registration that takes into consideration patient movement within six degrees of freedom. It employs lasers to determine patient position and as such is confined to surface images. Thus, treatment beams must be based relative to tattoos or other markers on a patient's skin, which have the problems mentioned above.
These existing alignment methods require an extensive amount of time to process complex diagnostic data, usually restrict accuracy to three degrees of freedom, limit patient movement, and make adjustment of either the treatment beams or the patient difficult. In addition, they are unable to generate instant reference images with which to compare the present position of a patient. They also require manual operations to supplement automatic procedures.